The present invention relates to a phase locked loop circuit and, more specifically, relates to a phase locked loop circuit with a lock detector that is based on a replica charge pump and a method of determining a lock condition, and a phase locked loop circuit with a lock detector that is based on a replica charge pump.
FIG. 1 illustrates a related-art lock detector commonly used in an integer/fractional-N phase lock loop (PLL). In such design, the lock detector 1 has two D-type flip flops (DFF) 10 and 20. One of the DFFs samples a reference signal SREF at a rising edge of the signal SOUT outputted by a voltage controlled-oscillator or a frequency divider of the PLL while the other of the DFFs samples the signal SOUT at the rising edge of the reference signal SREF. When locked, the rising edges of reference signal SREF and the signal SOUT will align. Therefore, the outputs of both DFFs will go high and an output LOCKD generated by an AND gate 50 registers a high, flagging a lock condition. However, there is always certain time delay for the DFFs to sample their respective inputs. Therefore, both of their clock inputs must be delayed by a certain amount with respect to the inputs. The two delay blocks 30 and 40 provide required delays. The delays of the two delay blocks 30 and 40 are equal under ideal conditions and let these be td.
In practice, there may still be a small delay tl, between rising edges of reference signal SREF and the signal SOUT under the lock condition. Then, the delay for the delay block of a leading waveform must be adjusted to be td+tl while for delay block of a lagging waveform is maintained at td. Otherwise, the lock detector 10 may malfunction. In many cases, such as for a fractional-N PLL, the delay time tl is large and may dynamically change with the alternating division ratio. In such cases, it is very difficult to determine how much delay should be set for delay blocks 30 and 40. As a result, most often, the output LOCKD does not register a steady high level, having occasional low periods when locked.